Radio receivers process received signals to extract wanted signals. The antenna for a radio receiver receives signals across the spectrum in a wide bandwidth, including the wanted signal and possibly unwanted signals. The receiver can use filters to reduce the level of unwanted signals that appear outside the frequency range of the wanted signal.
The receiver can use an analogue-to-digital converter (ADC). The digitised signal may then be used by a digital processor of the receiving device. The digitised signal may contain communications such as voice communications, email communications, or text communications.
The analogue-to-digital converter converts the signal from an analog voltage or current into a digital representation. By necessity the ADC has a limited digital range. An instantaneous voltage or current with higher amplitude than the maximum value allowed by the ADC is clipped. Clipping is an undesirable outcome. Further, the ADC component introduces unwanted noise. A wanted signal too low in amplitude has its signal-to-noise ratio reduced by the ADC noise.
To ensure that the noise introduced by the ADC does not deteriorate the signal-to-noise ratio of the wanted signal below a minimum value, the receiver must amplify or attenuate the received signal by an amount that depends both on the power of the wanted signal, and the noise floor of the ADC.
The received signal can contain a wanted signal, and one or more unwanted signals. In the most critical case, the power of the unwanted signals is significantly higher than the power of the wanted signal. Further, the frequency difference between wanted and unwanted signals is small, preventing effective removal of the unwanted signals by filtering prior to the ADC. It follows, that the unwanted signal appears at the input of the ADC with a power that is still considerably higher than the wanted signal. To prevent clipping distortion caused by the unwanted signal in the ADC, the prior amplification or attenuation of the signal must be chosen in such a way by an analog gain control algorithm (AGC), that the unwanted signal never exceeds the clipping level of the ADC.
Existing methods include choosing the gain or attenuation prior to the ADC in such a way, that a substantial margin between the power of the received signal and the clipping level of the ADC is maintained (headroom). The ADC can be switched to a higher performance mode, when the presence of blockers is detected. Further, a filter with high selectivity may be used to achieve sufficient rejection of unwanted signals prior to the ADC. The main disadvantage of these methods is that better performance is required from ADC and filters, leading to increased cost due to larger integrated circuit size and increased power consumption. Further, the achieved signal-to-noise ratio may be suboptimal, possibly reducing system throughput.
There is a desire for an improved method of controlling gain and ADC dynamic range in a radio receiver.
It is an object of the present invention to provide a method of controlling the gain of a receiver for an ADC or controlling the dynamic range of the ADC to ensure that received signals are optimally processed, or to at least provide a useful alternative.